Several precious metal-bearing ores, such as many gold and silver-bearing ores, are known to be refractory to standard recovery techniques such as cyanidation. An ore can be refractory due to the presence of sulfide minerals with which the precious metal is associated and from which the precious metal is difficult to separate. Typically, refractory sulfide ores are treated by decomposing the sulfide minerals in order to release the precious metal for subsequent recovery. One process for treating refractory sulfide ores is to pressure oxidize the ore at elevated temperature and pressure under acidic conditions to oxidize sulfide sulfur in the sulfide minerals.
Another reason that an ore might be refractory is that the ore contains significant amounts of organic carbon that can adsorb the precious metal in competition with a recovery operation. For example, during cyanidation recovery, an ore is leached with a cyanide to form a cyanide complex with the precious metal in the ore. The precious metal-cyanide complex can be adsorbed onto activated carbon granules and the precious metal recovered from the activated carbon granules. When organic carbon is present in the ore, however, the organic carbon can compete with the activated carbon for adsorbing the precious metal-cyanide complex, thereby significantly reducing the amount of precious metal that is recovered. The adsorption of the precious metal by the organic carbon is often referred to as preg-robbing, because the organic carbon is robbing the precious metal from a precious metal pregnant solution.
The acidic pressure oxidation process that has been used for sulfide refractory ores has not been found to be satisfactory for sufficiently reducing the preg-robbing problem when organic carbon is present. One process that has been used to treat refractory organic carbon ores is to subject the ore to a chlorination treatment prior to gold recovery using a chlorine-containing material. Chlorination, however, can be very expensive and is not satisfactory when the ore is also refractory due to the presence of sulfide minerals that are associated with the gold.
One process that has been used to treat ores that are refractory due to the presence of both sulfide minerals and organic carbon is to oxidatively roast the ore to oxidize substantially all of both the organic carbon and the sulfide minerals. Roasting is expensive, however, because the ore must be dry ground. Also, when the ore contains arsenic, a significant amount of arsenic is volatilized during roasting and poses a significant environmental problem. A similar problem also occurs for mercury when mercury is present in the ore. Also, solid residues, or calcines, of the roasting process often contain significant amounts of toxic substances, such as arsenic, in a soluble form, which require substantial additional treatment for safe disposal.
A need exists for a relatively simple and inexpensive processes for treating gold-bearing ores that are refractory due to the presence of organic carbon, and especially for those ores that are also refractory due to the presence sulfide minerals associated with the gold.